Ciao amico! Come stai?
edit:
By default, Weaviate will do a Ranked Fusion Relative Score Fusion see here
We have a nice blog article on this, for instance:
The rankedFusion algorithm is the original hybrid fusion algorithm that has
been available since the launch of hybrid search in Weaviate.
In this algorithm, each object is scored according to its position in the results for the given search, starting from the highest score for the top-ranked object and decreasing down the order. The total score is calculated by adding these rank-based scores from the vector and keyword searches
There is also the Relative Score. There is a nice explanation in the source code:
// FusionRelativeScore uses the relative differences in the scores from keyword and vector search to combine the
// results. This method retains more information than ranked fusion and should result in better results.
//
// The scores from each result are normalized between 0 and 1, e.g. the maximum score becomes 1 and the minimum 0 and the
// other scores are in between, keeping their relative distance to the other scores.
// Example:
//
// Input score = [1, 8, 6, 11] => [0, 0.7, 0.5, 1]
//
// The normalized scores are then combined using their respective weight and the combined scores are sorted
Also from that blog article:
In contrast to rankedFusion , however, relativeScoreFusion derives each objects score by normalizing the metrics output by the vector search and keyword search respectively. The highest value becomes 1, the lowest value becomes 0, and others end up in between according to this scale. The total score is thus calculated by a scaled sum of normalized vector similarity and normalized BM25 score.
Also notice that the Result Set keyword part of the explanation will only appear if there is a key word match against your objects.
Let me know if this helps 
Thanks!
